1. Field of the Invention
The present invention relates to an optical amplifier, and more particularly, to an optical amplifier which equalizes the power of optical signals having different wavelengths.
2. Description of the Related Art
Optical amplifiers (OAs) are expected to be widely employed in future communications systems. Erbium doped fiber amplifiers (EDFAs) serve to periodically amplify an optical signal when a great amount of data is transferred over great distances via an optical fiber without regeneration, to compensate for attenuation of the optical signal caused by long-distance transmission.
Some new problems, however, occur, such as dispersion, during periodic amplification of an optical signal for long-distance transmission. Wavelength division multiplexing (WDM) represents a method of overcoming some of these problems. In WDM, a great amount of data is transmitted over several carriers, each with a different wavelength, and thus transmission speed and capacity are increased.
Assuming that an optical carrier represents one channel, optical power representing the strength of signals can evolve differently in different channels. These power differences can be very large, if the signals are attenuated and re-amplified repeatedly in the optical amplifier, or if they travel through different paths in an optical network.
The power differences can stem from the following reasons:
(1) The gains can be different in different channels. A further difficulty arises in that if the gain of an optical amplifier such as an EDFA is homogeneously broadened and changed, the gains at different wavelengths change by different amounts. Here, the homogeneous broadening represents filling ideal atom positions with all ions in a gain medium, i.e., branching each ion into an ideal energy level by the Stark effect. Furthermore, it can be difficult or even impossible to know which level of gain an OA will operate at, since the gain level may vary with time. Still, EDFAs that are gain-flattened or gain-equalized regardless of wavelengths and channels have been demonstrated, including those that are gain-flattened or gain-equalized independently of operating conditions. However, the gain will not be ideally flat or equal. In systems with many concatenated OAs, even small gain differences between channels can be detrimental, and lead to significant power differences; and PA1 (2) The signal attenuation due to loss between amplifiers can be different in different channels, resulting in significant power differences. As for the amplification, attenuation can also vary with time, and this variation in attenuation can be different in different channels or wavelengths in an unpredictable way. PA1 (1) The signal powers applied to the system may be different at different wavelengths; PA1 (2) Different signals may travel through different channels in a complex network with routing. When the channels are combined again, their powers will most likely be different from each other, unless some form of power control is employed for each individual channel; and PA1 (3) Tunable optical taps may be used, which may attenuate the channels selectively in an unpredictable way.
It can be concluded that it is very unlikely that the gain will compensate for the attenuation at several wavelengths simultaneously for the majority of operating conditions (in contrast, for single wavelength systems, this occurs automatically at some wavelength so long as the loss does not exceed the gain available from the OAs). This is especially difficult since the attenuation between amplifiers conceivably changes with different wavelength dependencies for different reasons. Examples of the reasons can be splice degradation, incorporation of power splitters or other optical elements into the so transmission path, incorporation of dispersion compensating fibers, and increased micro-bending losses. In fact, with such an uncertainty in prediction of signal powers due to the dependence of the loss of the signal powers on the wavelengths, it is impossible to ensure a flat gain as the inter-amplifier loss changes, with homogeneously-broadened amplifiers like the EDFA.
Even if the gain and loss were always balanced for all channels, i.e., even if the sum of the gain and loss were 0 dB for all channels, this does not ensure that the powers in all channels would be equal. Unequal powers can still result for the following reasons:
For many applications, it would be better if the OAs could make the power of the different channels equal (automatic power equalization) rather than make the gain equal. At least, power differences should be kept within certain bounds. This requires that the gain of a channel with a low input power outside the bounds should be higher than the gains of channels with powers inside the bounds.
Commercially available EDFAs cannot equalize the power differences between WDM channels because the gain of the EDFAs is homogeneously broadened at room temperature (normal temperature). As a consequence, the gain at one wavelength is almost the same as the gains at all other wavelengths. Thus, it cannot be said that the gain of a high power channel is smaller than that of a low power channel. In other words, gains depend on the wavelengths of the channels.
In contrast, in a non-homogeneously broadened amplifier, the gain at one wavelength is partially independent of the gains at other wavelengths. Here, the non-homogeneous broadening means that a Stark branch changes for each individual lasing ion. In long distance WDM, provided that the gain at other wavelengths is not affected, at least to some extent, the signal gain at one wavelength is reduced if the signal power at that wavelength becomes large. This is termed gain compression or gain saturation. On the other hand, if there is a strong signal compressing the gain at another wavelength, the gain can remain high at the first wavelength.
Several methods have been proposed to equalize the inter-WDM power differences. One method relies on the cooling of a gain medium, i.e., an EDF (erbium doped fiber), to very low temperatures. An erbium gain can be essentially and non-homogeneously broadened by cooling the EDF to a liquefied nitrogen temperature, resulting in a reduction in the uniform erbium line-width. While this method is reported to work quite well, the added complexity in devices resulting from the cooling is a significant drawback.
In another method, the erbium gain can remain essentially and homogeneously broadened, and the EDFA gain can be non-homogeneously broadened as a whole by amplifying other signal wavelengths in other portions of the EDFA. Thus, the EDF can operate at room temperature. In a method using a twin-core EDFA, as an example, paths traversed by different wavelengths are spatially separated, and a gain medium is thus effectively non-homogeneously broadened as a whole, although each and every point in the gain medium is predominantly homogeneously broadened. This method also suffers from some drawbacks. The twin-core EDFA is known to generate more noise than that of a single-core EDFA, an undesired polarization dependence may arise, considerable amounts of power are lost, and fabrication of the twin-core fiber can be difficult.
In yet another method, wavelengths for different channels are decoupled by wavelength-selective couplers (WSCs), and amplified in different EDFs. The gains of the different channels can thus be decoupled from each other, which corresponds to a non-homogeneous broadening. Drawbacks of this approach are that the amplifier becomes more complicated, and pumping power is not used in an effective way.